1. Field of the Invention
The present invention relates to a wireless communication method for transmitting broadcast control channel information by the rotation of a beam from a directional antenna of a base station, a base station, and a receiving terminal.
2. Description of the Related Art
In wireless communication systems, as a kind of control channel, broadcast control channels (perch channels) are generally provided. Base stations transmit information necessary for communications to receiving terminals (e.g., mobile terminals) using the broadcast control channels. Information transmitted to receiving terminals using broadcast control channels (information necessary for communications) is hereinafter referred to as broadcast control channel information.
First, a user turns on a receiving terminal. Then, the receiving terminal searches for a near base station, and receives the broadcast control channel information transmitted from the found base station. The receiving terminal obtains the broadcast control channel information. The receiving terminal transmits to the base station a signal indicating information which identifies the receiving terminal. The receiving terminal waits in such a state where it can receive a signal transmitted from the base station. Consequently, the receiving terminal can perform data communications with the base station.
Accordingly, the receiving terminal needs to be capable of receiving the broadcast control channel information transmitted by the base station wherever the receiving terminal is within the area covered by the base station. Therefore, the base station transmits the broadcast control channel information with such a transmission power (e.g., radio wave intensity) that the receiving terminal can receive the broadcast control channel information whatever the receiving terminal is location with in the coverage area of the base station.
FIG. 1 is a diagram showing the structure of broadcast control channel information of a wireless communication system using the W-CDMA method. The broadcast control channel information has, for example, 64 blocks as shown in FIG. 1. Note that the after-mentioned expression “all the blocks of the broadcast control channel information” means one or more blocks constituting one piece of broadcast control channel information. In the case of FIG. 1, it means blocks 0 to 63. In addition, one block period (time needed by a base station to transmit one block) is approximately 20 msec.
Each block has data indicating an SFN (system frame number) and broadcast control block data (information necessary for communications). The SFN is data necessary for notifying receiving terminals (mobile terminals or the like) of the current system time. The SFN is represented by an integer.
One period of the SFNs is longer than a period of the broadcast control channel information (1280 msec). One period of the SFNs means the time period corresponding to (N−1) blocks when the SFN contained in the first block is zero and the SFN contained in the Nth block is also zero.
One period of the SFNs is an integral multiple of one period of the broadcast control channel information. Accordingly, by obtaining the SFN, the receiving terminal can determine the block number of the broadcast control block data corresponding to the SFN (information indicating where the broadcast control block data is in the broadcast control channel information). Note that the broadcast control block data corresponding to an SFN means the broadcast control block data contained in the same block in which the SFN is contained.
Consequently, the receiving terminal can recognize information necessary for communications based on transmitted blocks even if the block first received by the receiving terminal is not the same first block as when the receiving terminal started receiving the broadcast control channel information. The broadcast control channel information constituted as previously described is repeatedly transmitted from the base station across the coverage area thereof.
In order to reduce power consumption, the receiving terminal performs the following process. Once having obtained all the blocks of broadcast control channel information, the receiving terminal stores them in a built-in storage unit. Then, the receiving terminal reads out the broadcast control channel information from the storage unit as necessary.
Here, there may be cases where the contents of the broadcast control channel information transmitted from the base station varies with time. Therefore, among the blocks of the broadcast control channel information, specific blocks (e.g., blocks having block numbers of multiples of four) contain information only indicating, for example, “update of the Nth block.” These specific blocks are hereinafter referred to as update notification blocks. Accordingly, once having obtained all the blocks of the broadcast control channel information, the receiving terminal receives only update notification blocks thereafter. And, if the blocks are not updated, the receiving terminal waits until the next transmission timing for update notification blocks.
Thus, the power consumption of the receiving terminal can be reduced. Consequently, the usable time of the battery of the receiving terminal can be elongated. Note that, in the case where information indicating the update of the broadcast control channel information is contained in the update notification blocks, the receiving terminal receives an updated block (hereinafter referred to as the update block) based on the contents of the update notification blocks. Then, from among the blocks stored in the storage unit, the receiving terminal rewrites the block corresponding to the update block with the received update block.
FIG. 2 is a flowchart showing a method for receiving and storing the broadcast control channel information using a conventional receiving terminal. First, the receiving terminal receives a synchronizing signal necessary for receiving the broadcast control channel signal (signal indicating broadcast control channel information) transmitted from a base station. The receiving terminal judges whether synchronization necessary for receiving a broadcast control channel signal is achieved based on the synchronizing signal (S200). If synchronization has been not achieved, the process of step S200 is performed. On the other hand, if synchronization has been achieved, the process of step S210 is performed.
At step S210, the receiving terminal receives the broadcast control channel information transmitted from the base station. Then, the receiving terminal judges whether a completion flag is ON (S220). The completion flag is information indicating whether all the blocks of the broadcast control channel information have already been stored in the built-in storage unit of the receiving terminal.
(1) If the completion flag is ON at step S220, the process of step S230 is performed. The receiving terminal obtains and decodes the SFN contained in each block of the received broadcast control channel information. The receiving terminal judges whether the SFN is a multiple of four based on the decoded result of the SFN (S230). If the SFN is not a multiple of four, the process of step S200 is performed. On the other hand, if the SFN is a multiple of four, the receiving terminal decodes the broadcast control block data corresponding to the SFN. Then, the receiving terminal judges whether there is an updated block based on the decoded result (S240).
If there is no updated block, the process of step S200 is performed. On the other hand, if there is an updated block, the receiving terminal deletes, from the storage unit, the block which has the same number as that of the updated block (S250). Then, the receiving terminal sets the completion flag to OFF (S260). The OFF setting of the completion flag indicates that at least one block of all the blocks of the broadcast control channel information has been stored in the storage unit. Then, information concerning the completion flag is stored in the storage unit. Subsequently, the process of step S200 is performed.
(2) If the completion flag is OFF at step S220, the receiving terminal decodes the SFN and the block contents (e.g., broadcast control block data) contained in each block of the received broadcast control channel information (S270). The receiving terminal stores each block in the storage unit such that the block corresponds to the block number (S280). The receiving terminal judges whether all the blocks of the broadcast control channel information have been stored in the storage unit (S290). If all the blocks have not been stored, the process of step S200 is performed. On the other hand, if all the blocks have been stored, the receiving terminal sets the completion flag to ON (S300). The ON setting of the completion flag indicates that all the blocks of the broadcast control channel information are stored in the storage unit. Then, information concerning the completion flag is stored in the storage unit. Subsequently, the process of step S200 is performed. FIG. 3 is a diagram showing one example of information stored in the storage unit (information concerning the blocks and the completion flag).
In wireless communication systems using wireless communication methods, such as CDMA, broadcast control channels are also prepared similarly to the above-described case. However, in such wireless communication methods as CDMA, user identification processes and channel identification processes are performed by means of spreading codes. Accordingly, such wireless communication methods have, for example, a feature in that interference affects communication quality. Hereinafter, the above-described feature will be outlined using drawings.
FIG. 4 is a view showing the configuration of a conventional wireless communication system. A wireless communication system has base stations, and receiving terminals (e.g., mobile terminals), each of which exists in an area covered by one base station (such an area is hereinafter referred to as a coverage area) and which receives various types of channel information from the base station. In FIG. 4, a wireless communication system having three base stations is shown as an example.
The base stations 11a, 11b, and 11c transmit broadcast control channel information using broadcast control channels A, B, and C, respectively. Each base station transmits broadcast control channel information using broadcast control channel A, B, or C with such a transmission power that the broadcast control channel information can reach the entire coverage area thereof.
However, broadcast control channel information is also necessary when handover control is performed. Accordingly, there are cases where broadcast control channel information is transmitted to places near and outside a coverage area. In FIG. 4, a receiving terminal 12b communicates with the base station 11b through communication channel 1. In addition, according to the characteristics of CDMA, all of the base stations 11a, 11b, and 11c communicate with receiving terminals using the same frequency. Therefore, the receiving terminal 12b inevitably receives not only a signal of communication channel 1, which is a desired wave signal, but also signals of broadcast control channels A, B, and C as interference signals (undesired signals).
The communication quality in a wireless communication method using CDMA depends on the ratio of desired wave power to interference power (SIR). Accordingly, an increase in an interference signal lowers the communication quality, causing an increase in required transmission power. Moreover, the capacities of communication channels are wasted. Therefore, the reduction of interference signals is an important issue.
For example, broadcast control channel B is a channel necessary for various kinds of control. However, the broadcast control channel information transmitted using broadcast control channel B becomes interference for the receiving terminal 12b. Here, the phrase “the broadcast control channel information becomes interference for the receiving terminal 12b” means that the signal indicating the broadcast control channel information has a nonnegligible level of intensity compared to a signal desired by the receiving terminal. Therefore, preventing situations where broadcast control channel information becomes interference for receiving terminals is an important issue.
Accordingly, heretofore, broadcast control channel information has been transmitted to receiving terminals which exist in the coverage area of the directional antennas of a base station, by the rotation of a beam having a beam angle smaller than a coverage angle (e.g., sector angle), which is an angle between two boundary lines of the coverage area.
According to this technology, broadcast control channel information is intermittently transmitted to receiving terminals by the rotation of a beam having a small beam angle. In addition, the base station transmits communication channel information across the coverage area thereof, for example, using a nondirectional antenna. Thus, while a receiving terminal is receiving the communication channel information, the broadcast control channel information is being intermittently transmitted to the receiving terminal. Consequently, situations where the broadcast control channel information becomes interference for the receiving terminal are prevented.
However, in the above-described conventional technology, there has been the following problem. If the beam angle of a beam containing the broadcast control channel information is set to be smaller than the coverage angle, the broadcast control channel information is transmitted to the receiving terminal not continuously but intermittently.
Accordingly, for example, in the case where the total block number of the broadcast control channel information, the coverage angle, a beam rotation angular velocity, and the like do not have appropriate values, there arises a situation where the receiving terminal cannot receive at least one block from among all the blocks of the broadcast control channel information. This problem will be described in detail below.
Description will be made taking as an example the case where the broadcast control channel information has a plurality of blocks. A case where the receiving terminal receives one block during the period in which the base station transmits ten blocks (blocks contained in the broadcast control channel information) is considered. Moreover, the total block number of the broadcast control channel information is assumed to be 64. In this case, if the receiving terminal remains stationary, some blocks permanently cannot be received by the receiving terminal.
Specifically, the following case is considered: the block number of the block first received by the receiving terminal is 0, the block number of the block subsequently received by the receiving terminal is 10, and the block number of the block subsequently received by the receiving terminal is 20. In this case, as the receiving terminal repeatedly receives one block during each period in which the base station transmits ten blocks, the receiving terminal receives up to the 54th block. Thereafter, the receiving terminal receives the 0th block again. Therefore, the receiving terminal permanently cannot receive the blocks having odd block numbers.